Ruger AR‑556 MPR (.450 Bushmaster, 18.63″ Barrel, 5+1) Specifications Caliber: .450 Bushmaster (9.48 mm / .452″) Barrel: 18.63″ cold-hammer-forged 4140 chrome-moly with 5R rifling, 1:16 twist, threaded 11/16″–24 and equipped with Ruger’s radial-port muzzle brake Gas System: Carbine-length direct impingement Receiver & Handguard: Forged 7075-T6 upper, free-float 15″ M-LOK handguar... https://www.prime-firearms.com/product/ruger-ar-556-mpr-450-bushmaster-18-63-51/